Anode active material and lithium secondary battery including the same

This patent document claims the priority and benefits of Korean Patent Application No. 10-2021-0122147 filed at the Korean Intellectual Property Office (KIPO) on Sep. 14, 2021, the entire disclosure of which is incorporated herein by reference.

This patent document generally relates to an anode active material and a lithium secondary battery including the same.

The rapid growth of electric vehicles and portable devices, such as camcorders, mobile phones, and laptop computers, has brought increasing demands for secondary batteries which can be charged and discharged repeatedly.

Examples of the secondary batteries include lithium secondary batteries, nickel-cadmium batteries, and nickel-hydrogen batteries. The lithium secondary batteries are now widely used due to certain advantages over other types of batteries, including, e.g., high operational voltage and energy density per unit weight, a high charging rate, a compact dimension, etc.

A lithium secondary battery may include an electrode assembly including a cathode, an anode and a separation layer (separator), and an electrolyte immersing the electrode assembly. The lithium secondary battery may further include an outer case having, e.g., a pouch shape for accommodating the electrode assembly and the electrolyte.

The technology disclosed in this patent document can be implemented in some embodiments to provide an anode active material having improved life-span property.

The technology disclosed in this patent document can also be implemented in some embodiments to provide a lithium secondary battery including an anode active material having improved life-span property.

In some embodiments of the disclosed technology, an anode active material includes a core portion including a solid electrolyte, and a shell portion encapsulating the core portion and including a silicon-based active material.

In some embodiments, the shell portion may completely surround a surface of the core portion.

In some embodiments, the solid electrolyte may contain an oxide-based solid electrolyte or a sulfide-based solid electrolyte.

In some embodiments, the oxide-based solid electrolyte may include at least one selected from the group consisting of an LIPON compound, a perovskite-based compound, a NASICON compound, a garnet-type compound, glass, a phosphoric acid-based compound and a crystalline oxide. In some implementations, the term “LIPON compound” can be used to indicate a lithium phosphorus oxynitride compound. In some implementations, the term “NASICON” can be used to indicate a sodium (Na) super ionic conductor.

In some embodiments, the sulfide-based solid electrolyte may include at least one selected from the group consisting of a thio-LISICON compound, an LGPS compound, an LPS compound, 30LiS·26BS·44LiI, 63LiS·36SiS·1LiPO, 57LiS·38SiS·5LiSiO, 70LiS·30PS, 50LiS·50GeS, LiS—PS, LiS—SiS, LiI—LiS—SiS, LiI—SiS—PS, LiI—LiBr—LiS—PS, LiI—LiS—PS, LiI—LiO—LiSPS, LiI—LiS—PO, LiI—LiPO—PS, LiS—PS—GeS, LiS—PS—LiCl, LiI—LiS—BS, LiPO—LiS—SiS, LiPO—LiS—SiSand LiPO—LiS—SiS.

In some embodiments, an ionic conductivity of the solid electrolyte may be 1×10S/cm or more.

In some embodiments, the solid electrolyte may have an average particle diameter (D) in a range from 1 μm to 10 μm.

In some embodiments, the silicon-based active material may include a silicon particle, a silicon-carbon composite, silicon oxides and/or a silicon alloy.

In some embodiments, the shell portion may have a thickness in a range from 0.5 μm to 10 μm.

In some embodiments, the shell portion may further include a carbon-based active material.

In some embodiments, a content of the carbon-based active material contained in the shell portion may be in a range from 2 parts by weight to 20 parts by weight based on 100 parts by weight of the shell portion.

In some embodiments, the carbon-based active material may include activated carbon, carbon nanotube (CNT), carbon nano-wire, graphene, carbon fiber, carbon black, graphite, hard carbon, soft carbon and/or porous carbon.

In some embodiments, the shell portion may further include a carbon coating layer.

In some embodiments of the disclosed technology, a lithium secondary battery includes a case and an electrode assembly accommodated in the case. The electrode assembly includes an anode including the anode active material for a lithium secondary battery according to the above-described embodiments, and a cathode facing the anode.

In some embodiments, the lithium secondary battery may further include a liquid electrolyte injected into the case.

In an anode active material according to exemplary embodiments of the disclosed technology, a core portion including a solid electrolyte may be completely included within a shell portion including a silicon-based active material and may not be exposed to an outside at an initial stage of a life-span. As the life-span progresses, cracks may occur in the shell portion including the silicon-based active material, and the solid electrolyte of the core portion may newly participate in a reaction. Accordingly, a consumed electrolyte may be replenished to improve life-span properties of the lithium secondary battery.

With increasing demand for high-capacity, high-power lithium secondary batteries, a silicon-containing active material having a high capacity is being applied as an anode active material. The silicon-based active material has a large theoretical capacity, but a large volume expansion may occur while the lithium secondary battery is being charged. Such a volume expansion can cause cracks in the active material and side reactions of the electrolyte may occur on the cracked surface.

According to exemplary embodiments of the disclosed technology, an anode active material includes a core portion containing a solid electrolyte, and a shell portion capsulating the core portion and containing a silicon-based active material. In addition, an anode for a lithium secondary battery and a lithium secondary battery include the anode active material.

An anode active material for a lithium secondary battery using a silicon-based active material may have a remarkably greater theoretical capacity than that of a graphite-based active material. However, the silicon-based active material may have a large volume expansion ratio while being charged, and thus cracks may occur due to repeated expansion and contraction during repeated charge/discharge.

Accordingly, a surface area of the anode active material may be increased, and an electrolyte included in a battery may react with a newly increased surface area to be rapidly consumed. Further, side reactions such as a gas generation may occur during the reaction, thereby degrading life-span properties of the battery.

According to embodiments of the disclosed technology, the anode active material may include a core portion including a solid electrolyte, and a surface of the core portion may be encapsulated by a silicon-based active material to form a shell portion. In some embodiments, the shell portion may be formed to completely surround the surface of the core portion.

is a schematic cross-sectional view illustrating an anode active material according to exemplary embodiments.

Referring to, in an initial stage of life-span, a core portionincluding the solid electrolyte may be completely covered or surrounded by a shell portionincluding the silicon-based active material and may not be exposed to an outside (see (a) of). When cracksis generated in the shell portionas charging/discharging proceeds through a battery reaction, the solid electrolyte of the core portionpresent within an anode active material particle may newly participate in the reaction (see (b) of). Accordingly, the electrolyte consumed during the charging/discharging may be replenished to improve the life-span properties of the lithium secondary battery.

In exemplary embodiments, the solid electrolyte may include an oxide-based solid electrolyte and/or a sulfide-based solid electrolyte. In a preferable embodiment, the sulfide-based solid electrolyte may be used.

Non-limiting examples of the oxide-based solid electrolyte may include a LIPON compound such as LiPON, etc., a perovskite compound such as LaLiTiO(LLTO), etc., a NASICON compound such as LiAlTi(PO)(LATP), etc., a garnet type compound such as LiLaZrO(LLZO), etc., a glass such as 50LiSiO·50LiBO, a phosphoric acid-based compound such as LiAlTi(PO), LiAlGe(PO), etc., and a crystalline oxide such as LiSiPO. These may be used alone or in combination thereof.

Non-limiting examples of the sulfide-based solid electrolyte may include a thio-LISICON-type compound such as LiGePS, etc., an LGPS-type compound such as LiGePS, LiGePS, etc., an LPS-type compound such LiPS, LiPS, LiPS, etc., 30LiS·26BS·44LiI, 63LiS·36SiS·1LiPO, 57LiS·38SiS·5LiSiO, 70LiS·30PS, 50LiS·50GeS, LiS—PS, LiS—SiS, LiI—LiS—SiS, LiI—SiS—PS, LiI—LiBr—LiS—PS, LiI—LiS—PS, LiI—LiO—LiSPS, LiI—LiS—PO, LiI—LiPO—PS, LiS—PS—GeS, LiS—PS—LiCl, LiI—LiS—BS, LiPO—LiS—SiS, LiPO—LiS—SiS, LiPO—LiS—SiS, etc. These may be used alone or in combination thereof.

In some embodiments, an ionic conductivity of the solid electrolyte may be 1×10S/cm or more. Preferably, the ionic conductivity of the solid electrolyte may be 5×10S/cm or more, more preferably 1×10S/cm or more. In the above range of the ionic conductivity, an ion transfer between the anode and the cathode may be promoted to reduce an internal resistance between the anode and the cathode. Accordingly, power properties of the lithium secondary battery may be improved.

The ionic conductivity may be measured at room temperature, e.g., about 25° C. using a DC polarization method, or may be measured using a complex impedance method.

In some embodiments, an average particle diameter (D) of the solid electrolyte may be in a range from 1 μm to 10 μm, preferably from 3 μm to 7 μm. An appropriate amount of the solid electrolyte may be obtained in the particle size range, so that life-span property and energy density of the secondary battery may be improved.

The “average particle diameter (D50)” refers to a value of a particle diameter corresponding to 50% from a smallest particle based on 100% of the total number of particles in a cumulative distribution curve of particle sizes. The average particle diameter (D50) may be measured by a widely known method.

For example, the average particle size may be measured using a particle size analyzer, or may be measured from a TEM image or an SEM image. The average particle size may also be measured by a measurement using a dynamic light-scattering method, or may be calculated after counting the number of particles for each particle size range using a data analysis.

In some embodiments, the silicon-based active material may include a silicon particle, a silicon-carbon composite, a silicon oxide, a silicon alloy, or the like. These may be used alone or in combination thereof.

The silicon particles may be present in the form of primary particles or a secondary particle formed by agglomeration of the primary particles. The silicon-carbon composite may include silicon particles and/or carbon-silicon composite particles dispersed in a carbon matrix.

The silicon oxide may be represented as, e.g., SiOx (0<x<2). The silicon alloy may include, e.g., a Si—Z′ alloy (wherein Z′ is at least one element selected from an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, a rare earth element, or a combinations thereof, and Si may be excluded therefrom).

In some embodiments, a thickness of the shell portion may be in a range from 0.5 μm to 10 μm, preferably from 0.7 μm to 7 μm, more preferably from 1 μm to 5 μm. For example, if the thickness of the shell portion is less than 0.5 μm, cracks may occur in the shell portion in a pressing process for fabricating an electrode and the solid electrolyte in the core portion may be exposed.

For example, if the thickness of the shell portion exceeds 10 μm, a volume expansion ratio of the anode active material may be excessively increased, and cracks may easily occur due to repeated charging and discharging. In this case, electrical contact properties between the anode active materials may be deteriorated, or electrolyte consumption may be accelerated through the cracks. Further, an amount of the silicon-based active material may be excessively increased, and thus a consumed amount of the electrolyte may be greater than a replenished amount of the solid electrolyte.

In exemplary embodiments, the shell portion may further include a carbon-based active material. Accordingly, electrical conductivity and durability of the anode active material may be enhanced, and thus the power and life-span properties of the lithium secondary battery may be improved.

Non-limiting examples of the carbon-based active material may include graphite particles having artificially formed pores, or a carbon body (pyrolytic carbon) manufactured by fining a carbon precursor such as pitch to form pores therein.

In some embodiments, the carbon-based active material may include an amorphous structure or a crystalline structure. Preferably, the carbon-based active material may have an amorphous structure.

In some embodiments, the carbon-based active material may include activated carbon, carbon nanotube (CNT), carbon nano-wire, graphene, carbon fiber, carbon black, graphite, hard carbon, soft carbon, porous carbon (micro/meso/macro porous carbon), etc. These may be used alone or in a combination thereof.

In exemplary embodiments, the shell portion may further include a carbon coating layer. In this case, direct exposure of silicon in the silicon-based coating layer to an electrolyte solution may be prevented, and thus the side reaction with the electrolyte solution may be reduced. Accordingly, volume expansion during charging and discharging of the secondary battery may be suppressed, and the life-span properties of the lithium secondary battery may be further improved.

In some embodiments, a content of the carbon coating layer may be in a range from 2 parts by weight to 20 parts by weight based on 100 parts by weight of a total weight of the shell portion.

For example, a thickness of the carbon coating layer may be in a range from 0.001 μm to 10 μm, preferably from 0.01 μm to 5 μm, and more preferably from 0.01 μm to 1 μm. In the above thickness range, the shell portion including the silicon-based active material may be prevented from a direct contact with the electrolyte.